Fleas tied to black death

October 28, 2011

Halloween is a great time to explore dark subjects, and plague is certainly a dark subject. Bubonic plague or "black death" ravaged Europe and killed off nearly half of the human population of Europe during the middle ages. There were so many dead that they were buried in huge pits.

Plague still exists today but can be cured with antibiotics. Scientists wondered whether the modern plague is the same as the one that ravaged Europe or some milder form. Since flu is constantly mutating, they wondered if plague does the same - and whether it could return some day with devastating consequences.

Using skeletons unearthed from a mass grave of British plague victims, researchers were able to extract DNA of the plague bacterium (Yersinia pestis) from victims' bones and teeth. From this they were able to sequence the genome and found that the plague that exists today is almost completely unchanged. That means that if antibiotics had existed six or seven hundred years ago, millions of lives could have been saved.

Plague is spread by the bites of the rat flea, Xenopsylla cheopsis. Without treatment, bubonic plague will kill about half of all infected people within days. Rat fleas are found not just on rats, but also on other rodents including mice, prairie dogs, chipmunks and ground squirrels.

When a flea bites an infected rodent, it picks up plague bacteria, which multiply in its digestive tract until the flea's stomach is blocked. The flea gets hungrier and hungrier because it cannot digest, so it bites its host animal more and more. It takes in host blood, and bacteria enter the blood. Since the flea's stomach is blocked, the flea regurgitates the blood back into the host animal. When this happens, the rodent is infected with plague and soon dies.

Ordinarily, rat fleas prefer to stay on rats and other rodents. However, when large numbers of rodents start dying off, the fleas move to other animals that are handy, including humans.

During the Middle Ages, people lived in generally filthy conditions, particularly in cities. Conditions were crowded, and sanitation was nonexistent. There were open latrines and garbage was tossed everywhere. This was a perfect environment for rats.

Something else was going on during this time as well. For whatever reason, years before the plague even appeared, religious leaders were arousing the public against cats, calling them servants of the devil. Possibly this is because cats are independent and like to wander around and yowl at night.

Many unmarried women who lived alone practiced herbal medicine, and they often kept cats for companions. They, and their cats, were frequently accused of witchcraft and killed. When the plague appeared, these women usually were not infected because their cats kept rats away from their homes. Of course, that made superstitious people even more sure that they were practicing witchcraft, so even more so-called witches and their cats were killed. This lead to the slaughter of much of Europe's cat population. Many dogs also were killed during this time. With the lack of predators, even more rats thrived.

Plague outbreaks come and go in waves. Weather is a factor, as is the death of host animals. There is always a certain percent of rats that are immune to plague. They carry it in their systems and keep it going for years until conditions are right for another outbreak.

There have been several large outbreaks of plague in various parts of the world over the years, including a widespread epidemic in China and India at the close of the 19th century. There was a plague outbreak in Los Angeles in the 1920s.

As recently as last year, a campground in Los Angeles County was closed because a ground squirrel tested positive for plague bacteria. About five to 10 cases are reported in the U.S. every year, usually in the southwest. Plague has not been found in Australia.

- Send your insect questions to Claire Stuart by email at buglady@wv.net or write her (with self-addressed stamped envelope) in care of Living Section, The Journal, 207 W. King St., Martinsburg, WV 25401.